The invention relates to a method and apparatus for installing a flowline near the waterbottom. More particularly it relates to a method and apparatus for lowering a flowline through a body of water and subsequently hinging over the flowline to a substantially horizontal position near the waterbottom.
Connecting a pipeline or flowline bundle to a subsea production facility can be a difficult and time consuming procedure, particularly in deep water. This is not difficult to understand once it is appreciated that the pipeline may be several hundred meters below the pipelay vessel, relatively immobile and difficult to remotely manipulate. A method for approaching this problem is the so-called flowline stab-in and hinge over technique disclosed in U.S. Pat. No. 4,676,696.
In accordance with this known technique a flowline is stabbed vertically into a guide tube which is mounted near the waterbottom, until a pair of pivots mounted near the lower end of the flowline land onto a pair of pivot supports which are located at the lower ends of a pair of vertical slots formed on the inner side of the guide tube. After the pivots have landed onto said supports, lowering of the flowline is continued while the pipelay vessel moves away from above the guide tube, thereby hinging over the flowline to a horizontal position and laying the flowline away from the guide tube.
A drawback of the known technique is that a heaving motion of the pipelay vessel may cause a rough landing of the pivots onto the pivot supports which may lead to buckling of the flowline which is suddenly stopped at its lower end while it is still being lowered at its upper end.